GABA occurs in 30-40% of all synapses-only glutamate is more widely distributed. Neurons in every region of the brain use GABA to fine-tune neurotransmission. Increasing GABA at the neuronal synapse inhibits the generation of the action potential of the neuron, thereby making it less likely to excite nearby neurons.
A single neuron may have thousands of other neurons synapsing onto it. Some of these release activating (or depolarizing) neurotransmitters; others release inhibitory (or hyperpolarizing) neurotransmitters. GABA is the primary inhibitory neurotransmitter, which means it decreases the neuron's action potential. When the action potential drops below a certain level, known as the threshold potential, the neuron will not generate action potentials and thus not excite nearby neurons.
The nucleus of a neuron is located in the cell body. Extending out from the cell body are dendrites and axons. Dendrites conduct impulses toward the cell body, Axons conducting impulses away from the cell body.
A recording electrode has been attached to a voltmeter to record the charge across the cell membrane, the thin layer that controls movement in and out of the neuron. The resting potential in excitable neurons is usually around -65 to -70 millivolts (mV), which can be seen on the voltmeter.
Excitatory synapses reduce the membrane potential:
The synapses labeled A, B, and C are excitatory (e.g. glutamate ACH). These synapses release activating neurotransmitters, which reduce the resting potential of the neuron.
If the voltage reaches the threshold potential, typically around -50 mv, an action potential is generated, which will travel down the axon, where it will communicate with a nearby cell.
The strength of the stimuli that produce an action potential is important only insomuch as it reaches threshold potential. The resultant action potential is always the same, whether it was created by relatively strong or relatively weak stimuli.
action potential is a constant.
Decreasing the action potential:
GABA is the primary inhibitory neurotransmitter, which means it decreases the neuronÃ¢Â€Â™s action potential. When the action potential drops below the threshold potential, the neuron will not excite nearby neurons.
Exitatory PostSynaptic Potential (EPSP):
The Exitatory PostSynaptic Potential (EPSP) of a single excitatory synapse is not sufficient to reach the threshold of the neuron. Rather, when a number of EPSPs are created in quick succession, their charges sum together. It is the combined sum of these EPSPs that creates an action potential
Activation of inhibitory synapses such as GABA, on the other hand, makes resting potential more negative. This hyperpolarization is called an inhibitory postsynaptic potential (IPSP). Activation of inhibitory synapses (D and E) makes the resting potential of the neuron more negative. The resulting IPSP may also prevent what would otherwise have been effective EPSPs from triggering an action potential.
It is the total summation of the EPSPs and IPSPs that determines whether a neuronÃ¢Â€Â™s charge is sufficient to cross the potential threshold.